Currently there are various types of artificial heart valves which have been proposed for clinical use. They include: (1) those made of a rigid metal frame with a central occluder which functions as a check valve with each beat of the heart, (2) tissue valves made from an animal's (pig) heart valve that is stretched and sewn over a rigid metal framework to provide central flow of blood, and (3) tri-leaflet valves of limited longevity and reliability. Such valves have never been clinically acceptable.
Central occluder valves consist, typically, of "ball-in-cage" or low-profile "disc-in-cage" designs utilizing plastic, silicone rubber, carbon or metal occluders. The rubber occluders suffer from two major shortcomings: (a) they are subject to "lipid" adsorption from the blood stream with concomitant changes in dimensions and physical integrity; splitting, tearing, and clotting result (see Bonnabeau, R. C. Jr., and Lillehei, C. W.: "Mechanical ball failure in Starr-Edwards Prosthetic Valves", J. Thorac. Cardiovas. Surg. 56: 258, 1968;
Hylen, J. C.: "Durability of Prosthetic Heart Valves", Am. Heart J. 81: 299, 1971; PA1 Hylen, J. C., Hodam, R. P. and Kloster, F. E.: "Changes in the Durability of Silicone Rubber in Ball-Valve Prostheses", Annals Thorac. Surg. 13: 324, 1972; PA1 Aston, S. J. and Mulder, D. G.: "Cardiac Valve Replacement, A Seven-Year Follow-up", J. Thorac. Cardiovas. Surg. 61: 547, 1971; PA1 Starr, A., Pierie, W. R., Raible, D. A., Edwards, M. L., Siposs, G. C., and Hancock, W. D.: "Cardiac Valve Replacement, Experience With the Durability of Silicone Rubber", Suppl. I to Circulation, 33, 34, April 1966) and PA1 (b) they possess a low order of wear resistance (see Boretos, J. W., Detmer, D. E. and Donachy, J. H. "Segmented Polyurethane: A Polyether Polymer, II, Two Years Experience", J. Biomed. Mater. Res. 5: 373, 1971). PA1 Clinical evidence with the "low profile" design exemplifies this characteristic, (Detmer, D. E.; McIntosh, C. L.; Boretos, J. W.; Braunwald, N. S.: "Polypropylene Poppets for Low-Profile Prosthetic Heart Valve", Annals Thorac. Surg. 13: 122, 1972). PA1 Ablaza, S. G. G., Blanco, G., Javan, M. B., Maranhao, V. and Goldberg, H. "Cloth Cover Wear of the Struts of the Starr-Edwards Aortic Valve Prosthesis", J. Thorac. Cardiovas. Surg. 61: 316, 1971; PA1 Thomas, C. S., Killen, D. A., Alford, W. C., Burrus, G. R. and PA1 Stoney, W. S.: "Cloth Disruption in the Starr-Edwards Composite Mitral Valve Prosthesis", Annals Thorac. Surg. 15: 434, 1973). Cloth covering has been shown to minimize thrombus formation on metal frames and is generally considered necessary for these designs, (Detmer, D. E. and Braunwald, N. S. "The Metal Poppet and the Rigid Prosthetic Valve", J. Thorac. Cardiovas. Surg. 61: 175, 1971; PA1 Ablaza, S. G. G., Blanco, G., Javan, M. B., Maranhao, V. and Goldberg, H. "Cloth Cover Wear of the Struts of the Starr-Edwards Aortic Valve Prosthesis", J. Thorac. Cardiovas. Surg. 61: 316, 1971). PA1 However, paravalvular leaks caused by cloth wear on the valve seats are responsible for high blood hemolysis, (Thomas, C. S., Killen, D. A., Alford, W. C., Burrus, G. R. and Stoney, W.: "Cloth Disruption in the Starr-Edwards Composite Mitral Valve Prosthesis", Annals Thorac. Surg. 15: 434, 1973). PA1 (a) leaflets are not readily torn or fatigued due to flexing during use; PA1 (b) the valve does not require the use of cloth, flocking or other textile forms for strength; PA1 (c) the possibility of abrasion due to bearing or rubbing surfaces is avoided; PA1 (d) leaflets are of a thin membrane structure and biaxially oriented for unusually high strength and durability while, at the same time, maintaining a gossimer-like characteristic for ease of operation at normal blood pressures; PA1 (e) the entire structure is made from one material, such as polyether polyurethane, thereby precluding the the possibility of interactions between dissimilar materials; PA1 (f) all three leaflets are basically one piece and cannot separate from each other or the frame to which they are attached; PA1 (g) the edges or lips of the leaflets are heavier than the main body of the leaflets and form a tight and durable seal at the commissure line of the valve with each closing cycle whereby flexing of the valve struts accomodates the complete and natural closing configuration and minimizes stress along the leaflet edges; PA1 (h) the junction between the leaflets and the frame is a smooth transition to obviate stress concentration and distribute it throughout; PA1 (i) radiating lines from the above junction add additional reinforcement for flexural strength and stress distribution without increasing bulk or stiffness; PA1 (j) valves of this design are not subject to being misshapened due to misalignment when sutured in place of the natural valve; PA1 (k) the base of the valve cannot be detached and is a permanent part of the valve; PA1 (l) the base can be of a rigid polurethane flange shape or a flexible porous or foam polyurethane construction or a combination of both, and can be covered with cloth or incorporate cloth padding to lend strength and ease of insertion for permanent fixation to the living tissues, heart wall or vessel. PA1 (a) allowing for uninterrupted central laminar flow of blood similar to that of the natural valve; PA1 (b) providing the leaftlets with a gossimer nature that offers very little resistance; PA1 (c) providing rapid opening and closing; PA1 (d) providing complete, unobstructed opening and complete non-regurgitant closing; PA1 (e) simulating the natural valve in size and shape without blocking adjacent blood vessel orifices when implanted; PA1 (f) providing struts that are rigid enough not to allow the leaflets to invert upon themselves even though a very low profile is used. PA1 (a) providing surfaces which are compatible with the blood and of a polyether urethane type; PA1 (b) providing surfaces which are free from tissue-in-growth or fibrous encapsulation; PA1 (c) providing a valve which is hydrolytically and enzymatically stable and does not degrade in the blood stream over extended periods of time; PA1 (d) providing surfaces which are smooth and clean and do not encourage calcium deposits on their surfaces; PA1 (e) obviating the formation of emboli to the brain or lungs by excluding release of fragments of textile materials from flexing areas of the valve; PA1 (f) using biocompatible polyurethanes of a polyether type which are free from absorption of body fluids which could alter them physically or chemically; PA1 (g) providing all surfaces that can be readily treated with anti-coagulants or other blood compatible materials. PA1 (a) readily sterilized by conventional methods; PA1 (b) manufactured in large numbers. Its structure can be varied in size according to patient needs and does not rely upon living donors; PA1 (c) constructed entirely from man-made materials and as such is suitable for use in artificial heart assist devices which must undergo conditions of complex assembly, sterilization and storage; PA1 (d) industrially producible to a high degree of duplication and reliability.
The metal and carbon occluders are: (a) abrasive to cloth covered struts causing fragmentation of the cloth with ensuing emboli, (Detmer, D. E. and Braunwald, N. S.: "The Metal Poppet and the Rigid Prosthetic Valve", J. Thorac, Cardiovas. Surg. 61: 175 1971;
The metal and carbon occluders are also: (b) abrasive to bare metal valve surfaces, especially for metal to metal contact, and (c) hard and they accordingly generate a "clicking" wound with each cycle. This noise and its associated anticipation is highly distressing to patients. In addition, in metal and carbon occluders (d) the central flow of blood is blocked by the presence of the occluder, reducing the potential volume output, increasing resistance to flow, and causing turbulance of flow which is believed to add to the incidence of thromboembolism.
Tissue valves are generally made by sewing excised pig valves over a rigid framework. These have the following disadvantages: (a) Tissue valves individually variable due to the fact that they are extracted from a living animal and subject to extremes of physical and physiological differences which have occured during growth of the animal. (b) There is no absolute way of testing individual valves for strength and durability prior to use. (c) Fixation techniques must be used to preserve tissue valves after extraction and during fabrication and storage and bacterial invasion has been difficult to control. Absolute sterility is difficult to assure, and patients occasionally become physically distressed due to bacterial endocarditis following surgical placement of a tissue valve. (d) Long-term implants have shown evidence of calcium deposits generated on the surface of the leaflets in some cases rendering them stiff and non-functional. (e) Tissue valves are not suitable for use in artificial heart assist devices where assembly, storage, and sterilization of the device must be done well in advance of surgery and under conventional sterilization techniques which would greatly impair or destroy living tissues.
Over the past 20 years a number of trileaflet valves has been constructed. Various materials and combinations of materials such as epoxies, silicones, Teflon, Dacron, and polyester polyurethanes have been used. For example, the patent to Sako et al, No. 3,940,802 discloses a non-toxic compound for use in heart valves, comprising 100 parts by weight PVC and 50-100 parts by weight of polyurethane. Generally, these valves have been clinically unsuccessful for the following reasons: Silicone-Dacron polyester valves of a trileaflet configuration require the cusps to be made thick, relative to normal tissue valves, to give them strength and shape. Unreinforced silicone rubber is too weak to be used alone. Unfortunately, the Dacron reinforcement prevents efficient opening and closing of the valve and significantly reduces the flexural fatigue resistance of the silicone. Fracture of the fibers results in premature failure. Poor performance in the form of regurgitation of blood is a common problem with such stiff valves.
Uncoated fabric valves of Dacron or Teflon have proven unsatisfactory because of the loss of function due to heavy fiberous tissue ingrowth which impairs opening and closing. Teflon has been shown to rapidly fragment due to fatigue failure.
Polyurethane tri-leaflet valves attempted in the past suffer from the following defects: (a) Leaflets detach from their mounting frames when assembled from individual leaflets. (b) Leaflets flex-fatigue at the mounting frame when the frame is rigid, concentrating the flexural strain pattern at a point along the rigid-flexible junction. (c) The thin leaflet construction, when unreinforced, has a propensity to tear along its commissure line; reinforced materials are too stiff to function. (d) Previous polyurethane leaflet valves were constructed of a hydrolytically unstable polyester polyurethane which rapidly degrades in the blood stream causing premature failure. (e) Thrombi generated on the surface were common occurrences with earlier designs using polyester polyurethanes. (f) Use of dissimilar materials can induce adverse reactions in designs where polyurethane leaflets are attached to an epoxy frame. In such cases, residual amines from the epoxy can migrate into the polyurethane causing degradation of physical properties and adverse surface characteristics may develop which can stimulate an inflammatory, toxic, or otherwise incompatible condition.
The U.S. Pat. Nos. to Hancock, 3,755,823; Parsonnet, 3,744,062, and Kischer, 3,548,417 are examples of prior art heart valves which suffer from one or more of the defects noted above.